System for multiple energy storage and management and method of making same

ABSTRACT

A propulsion system includes an electric drive, a first energy storage system electrically coupled to the electric drive through a DC link, and a second energy storage system electrically coupled to the first energy storage system in a series connection. The first energy storage system comprises a high specific-energy storage device; the second energy storage system comprises a high specific-power storage device. The propulsion system also includes a bi-directional boost converter electrically coupled to the first and second energy storage systems such that a terminal of the first energy storage system is electrically coupled to a low voltage side of the bi-directional boost converter and a first terminal of the second energy storage system is coupled to a high voltage side of the bi-directional boost converter. The series connection bypasses the bi-directional boost converter.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of and claims priorityto U.S. patent application Ser. No. 13/224,669 filed Sep. 2, 2011, whichclaims priority to U.S. patent application Ser. No. 12/539,056 filedAug. 11, 2009, now U.S. Pat. No. 8,026,638 issued on Sep. 27, 2011, thedisclosures of which are incorporated herein.

BACKGROUND

Embodiments of the invention relate generally to drive systems, and morespecifically to battery powered drive systems such as those used inbattery-powered electric vehicles or hybrid vehicles.

Recently, electric vehicles and hybrid electric vehicles have becomeincreasingly popular. These vehicles are typically powered by one ormore batteries, either alone or in combination with an internalcombustion engine. In electric vehicles, the one or more batteries powerthe entire drive system, thereby eliminating the need for an internalcombustion engine. Hybrid electric vehicles, on the other hand, includean internal combustion engine to supplement the battery power, whichgreatly increases the fuel efficiency of the vehicle.

Traditionally, the electric and hybrid electric propulsion systems inthese vehicles use large batteries, ultracapacitors, flywheels, or acombination of these elements so as to provide sufficient energy topower the electric motor. While generally effective, the size and weightof the elements reduced the overall efficiency of the propulsion systemand presented challenges for integration into the vehicles themselves.

Another challenge related to conventional electric propulsion systems isthat the nominal voltage of the energy storage units (i.e., batteriesand/or ultracapacitors) set the overall system voltage. Thus, the energyavailable to power the electric motor was limited to the energyavailable in the energy storage units themselves. Such a configurationlimits the overall reliability and efficiency of the electric propulsionsystem, as the voltage demands of the electric motor were often fargreater than the energy storage unit voltage. To combat this issue, abi-directional boost converter may be used to decouple the energystorage unit voltage from a direct current (DC) link voltage, whereinthe DC link is coupled to the electric motor. The bi-directional boostconverter acts to increase, or “boost”, the voltage provided from theenergy storage unit to the DC link to meet the power demands of theelectric motor. In fact, the ratio of the DC link voltage to the energystorage unit voltage is typically greater than 2:1. The bi-directionalboost converter enables such an increase in voltage supplied to the DClink without the need for an increase in the size of the energy storageunit or units.

While the bi-directional boost converter successfully allows for anincreased supply of voltage to the DC link without a correspondingincrease in size of the energy storage unit(s), the efficiency of thebi-directional boost converter degrades during certain operating modes.In particular, during high-speed and high-power acceleration anddeceleration of the vehicle, the ratio of DC link voltage to batteryvoltage is often greater than 2.5:1. Under these operating modes, thelevel of electrical current to which the components of the boostconverter are subjected is very high, and therefore there is asubsequent need for proper thermal design to dissipate heat in the powerelectronic components of the boost converter. This thermal cyclingstress on the components of the bi-directional boost converter mayreduce reliability as well as overall system efficiency.

Furthermore, during high-speed and high-power deceleration, a conceptknown as “regenerative braking” enables power at potentially relativelyhigh voltage generated by the electric motor to be cycled back throughthe bi-directional boost converter for storage in the energy storageunit(s). However, at high DC link voltage to battery voltage ratios,high losses within the bi-directional boost converter call for properheat dissipation in the electrical components. Also, the regenerationpower provided to the energy storage unit is often limited by the chargeacceptance of the energy storage unit itself, which further reduces theefficiency of the system.

Therefore, it is desirable to provide an electric and/or hybrid electricpropulsion system having greater overall system efficiency and a lowercost than traditional electric and hybrid electric propulsion systems.

BRIEF DESCRIPTION OF THE INVENTION

Embodiments of the invention provide a propulsion system that includesan electric drive and a first energy storage system electrically coupledto the electric drive through a direct current (DC) link, the firstenergy storage system comprising a high specific-energy storage device.The propulsion system also includes a second energy storage systemelectrically coupled to the first energy storage system in a seriesconnection, the second energy storage system comprising a highspecific-power storage device. The propulsion system further includes afirst bi-directional boost converter electrically coupled to the firstand second energy storage systems such that a terminal of the firstenergy storage system is electrically coupled to a low voltage side ofthe first bi-directional boost converter and a first terminal of thesecond energy storage system is coupled to a high voltage side of thefirst bi-directional boost converter. The series connection bypasses thefirst bi-directional boost converter.

In accordance with another aspect of the invention, a method ofassembling a control system for an electric drive includes providing afirst bi-directional boost converter and coupling a first terminal of ahigh specific-energy storage device to a low-voltage side of the firstbi-directional boost converter. The method also includes coupling afirst terminal of a high specific-power storage device to a high-voltageside of the first bi-directional boost converter, and coupling the highspecific-energy storage device in series with the high specific-powerstorage device such that the first terminal of the high specific-energystorage device is electrically coupled to a second terminal of the highspecific-power storage device.

In accordance with another aspect of the invention, an energy storagearrangement for an electrically powered system includes a firstbi-directional boost converter and a first energy storage systemelectrically coupled to a first side of the first bi-directional boostconverter, the first energy storage system comprising a highspecific-energy storage device. The energy storage arrangement alsoincludes a second energy storage system electrically coupled to a secondside of the first bi-directional boost converter, the second energystorage system comprising a high specific-power storage device. Further,the energy storage arrangement includes an electrical link forming aseries connection between a positive terminal of the first energystorage system and a negative terminal of the second energy storagesystem, wherein the electrical link bypasses the first bi-directionalboost converter.

Various other features and advantages will be made apparent from thefollowing detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate preferred embodiments presently contemplated forcarrying out the invention.

In the drawings:

FIG. 1 schematically illustrates a propulsion system according to anembodiment of the invention.

FIG. 2 schematically illustrates another embodiment of the propulsionsystem.

FIG. 3 schematically illustrates another embodiment of the propulsionsystem.

FIG. 4 schematically illustrates another embodiment of the propulsionsystem.

DETAILED DESCRIPTION

A system is shown to include an electric drive, a first energy storagesystem comprising at least a high specific-power energy storage device,such as an ultracapacitor, and a second energy storage systemelectrically coupled to the electric drive through a direct current (DC)link. Both the first energy storage system and the second energy storagesystem are electrically coupled to a multi-channel bi-directional boostconverter. Furthermore, the positive terminal of the high specific-powerenergy storage device is also coupled to the negative terminal of thesecond energy storage system to bypass the multi-channel bi-directionalboost converter. Such a connection between the high specific-powerenergy storage device and the second energy storage device enables ahigh voltage level to be provided to the electric drive duringacceleration, as well as an increased capability for energy capture inthe first energy storage system during regenerative braking events.

Referring to FIG. 1, a propulsion system 100 according to an embodimentof the invention is shown. Propulsion system 100 includes, in part, afirst energy storage system comprising an energy battery 102 and a highspecific-power energy storage device 104. Propulsion system 100 alsoincludes a multi-channel bi-directional boost converter 106. Highspecific-power energy storage device 104 may be, for example, anultracapacitor. In this case, an ultracapacitor represents a capacitorcomprising multiple capacitor cells coupled to one another, where thecapacitor cells may each have a capacitance that is greater than 500Farads. The term energy battery used herein describes a high specificenergy battery or high energy density battery demonstrated to achieve anenergy density on the order of 100 W-hr/kg or greater (e.g., a Li-ion,sodium-metal halide, sodium nickel chloride, sodium-sulfur, zinc-air,nickel metal halide, or lead acid battery, or the like). Energy battery102 and high specific-power energy storage device 104 are coupledtogether on a low-voltage side 202 of multi-channel bi-directional boostconverter 106, wherein a negative terminal 204 of energy battery 102 anda negative terminal 206 of high specific-power energy storage device 104are coupled to a bus 108, while a positive terminal 208 of energybattery 102 is coupled to a bus 110, which is a positive bus thatconnects through an inductor to one channel of multi-channelbi-directional boost converter 106 on the low-voltage side 202 ofmulti-channel bi-directional boost converter 106. A positive terminal210 of high specific-power energy storage device 104 is coupled to a bus220, which is coupled through an inductor on the low-voltage side 202 ata second channel (b) of multi-channel bi-directional boost converter106.

System 100 further includes a second energy storage system, whichcomprises an energy storage device 112, and an AC traction drive 212,which includes a DC-AC inverter 114 and an AC motor 116 coupled to ahigh-voltage side 214 of multi-channel bi-directional boost converter106. Energy storage device 112 may be, for example, a battery having ahigh specific-power rating. Alternatively, energy storage device 112 mayalso be an ultracapacitor. AC traction drive 212, in an alternativeembodiment, may be replaced by a DC traction drive (not shown) byreplacing inverter 114 with a DC chopper (not shown) and by replacing ACmotor 116 with a DC motor (not shown). Energy storage device 112 iscoupled with multi-channel bi-directional boost converter 106 via apositive DC link 118. DC-AC inverter 114 is also coupled to positive DClink 118 and a negative DC link 120, through which DC-AC inverter 114receives a DC voltage and then supplies an alternating current to ACmotor 116. Negative DC link 120 typically has the same potential as bus108 on low-voltage side 202 of multi-channel bi-directional boostconverter 106.

During typical operation, multi-channel bi-directional boost converter106 acts to boost the voltage provided by low-voltage side 202 of system100 to high-voltage side 214 of system 100, as well as to regulate thevoltage and provide over-current protection to energy battery 102, highspecific-power energy storage device 104, and energy storage device 112.While energy storage device 112 (or the combination of energy storagedevice 112 and high specific-power energy storage device 104) isgenerally capable of providing sufficient voltage to power the AC motor116 such that a vehicle may be operated at a relatively slow speed, thevoltage provided to the AC motor 116 during periods of increasedacceleration may need to be supplemented. In such instances, energy fromenergy battery 102 on low-voltage side 202 of multi-channelbi-directional boost converter 106 is utilized to provide the voltagenecessary for increased acceleration of the vehicle. Energy from energybattery 102 is used when the State of Charge (SOC) of highspecific-power energy storage device 104 is depleted below somepredetermined minimum value, typically a value below the voltage ofbattery 102. When the SOC of high specific-power energy storage device104 reaches this predetermined minimum value, a unidirectional couplingdevice 122 conducts such that the multi-channel bi-directional boostconverter 106 extracts energy primarily from energy battery 102 usingtwo channels of the multi-channel bi-directional boost converter 106,thereby allowing approximately twice the rated power compared to asingle channel of the multi-channel bi-directional boost converter 106.Unidirectional coupling device 122 is shown to be a diode in theembodiment of FIG. 1, but it is to be understood that unidirectionalconducting apparatus 122 could be implemented using other knowncomponents and circuit techniques. Such a configuration acts tofacilitate increasing the operation speed of the vehicle, particularlywhen the available energy of high specific-power energy storage device104 is depleted or near a predetermined voltage limit.

In the event high specific-power energy storage device 104 is at arelatively low SOC, or low voltage, energy battery 102 voltage can beboosted to the high side DC links 118 and 120 via low side (channel “a”)of multi-channel bi-directional boost converter 106 through positive bus110. The voltage provided by energy battery 102 through positive bus 110and/or high specific-power energy storage device 104 through a positivebus 220 is “boosted,” or increased, via the multi-channel bi-directionalboost converter 106 by a boost ratio typically greater than 2:1. In thisway, even with the output capabilities of energy battery 102 and/or highspecific-power energy storage device 104, the voltage and power neededto accelerate AC motor 116 may be provided due to the voltage-boostingcapabilities of multi-channel bi-directional boost converter 106. Inaddition, energy from the energy battery 102 may be utilized to chargeone or both of high specific-power energy storage device 104 and energystorage device 112 simultaneously via multi-channel bi-directional boostconverter 106.

While the operation of multi-channel bi-directional boost converter 106may be sufficient under normal operating conditions (e.g., lowacceleration and/or deceleration), the efficiency of multi-channelbi-directional boost converters such as multi-channel bi-directionalboost converter 106 may degrade during high acceleration or decelerationof the vehicle. That is, as there is an increase in the ratio of voltagerequired to sufficiently power an AC motor versus voltage available onthe respective low voltage sides of the multi-channel bi-directionalboost converter, a multi-channel bi-directional boost converter mayexperience increased electrical loss, leading to thermal cyclingstresses due to an increase in electrical current through components ofthe multi-channel bi-directional boost converter. These increasedcurrents may lower the efficiency of the bi-directional boost converter,which require proper thermal design and hardware to dissipate the heatfrom these losses in the power electronic components. However, theembodiment shown in FIG. 1 addresses this issue to greatly improve theefficiency of system 100, especially during operation at relatively highpower, high speed vehicle acceleration and deceleration.

Specifically, the positive terminal 210 of high specific-power energystorage device 104 is coupled in series with the negative terminal 216of energy storage system 112 via a link 124. Link 124 bypasses onechannel of multi-channel bi-directional boost converter 106 to enablethe voltage outputs of high specific-power energy storage device 104 andenergy storage device 112 to be summed, thereby utilizing the highspecific-power characteristics of high specific-power energy storagedevice 104 and energy storage device 112. During motoring events such aspulsed loads, steady state loads, vehicle cruise, and vehicleacceleration, the combined voltage of these two energy storage devicescan be used to provide sufficient voltage and power to AC motor 116without incurring losses related to passing current throughmulti-channel bi-directional boost converter 106. Additionally, couplinghigh specific-power energy storage device 104 and second energy storagedevice 112 in series enables fewer battery cells to be used as comparedto conventional traction battery systems having one or more tractionbatteries directly coupled to a DC link of an inverter or load, therebyreducing cost, weight, balancing, and reliability issues.

In addition to providing increased power capabilities for accelerationof the motor, the series connection of high specific-power energystorage device 104 and energy storage device 112 also provides forgreater efficiency for energy capture during regenerative brakingevents. Unlike energy battery 102, both high specific-power energystorage device 104 and energy storage device 112 are operable at a lowSOC and are capable of rapid high power electrical charge acceptance. Assuch, these energy storage devices are capable of accepting much of theregenerative power from the high voltage regenerated energy generated byAC motor 116 during overhauling loads such as vehicle deceleration.During such regenerative braking events, regenerative energy can beefficiently stored in high specific-power energy storage device 104 andenergy storage device 112, again without incurring the losses associatedwith the limitations of multi-channel bi-directional boost converter106, as link 124 enables the bypass of multi-channel bi-directionalboost converter 106. The energy stored in high specific-power energystorage device 104 and energy storage device 112 can then be used forsubsequent accelerations, which again improves the overall efficiency ofthe entire propulsion system 100.

Yet another advantage to the exemplary embodiment of FIG. 1 is theability to dynamically control the energy levels provided to and fromthe energy storage devices. Multi-channel bi-directional boost converter106 is operable as an Energy Management System (EMS) to adaptivelycontrol these energy levels based on parameters such as vehicle speed,AC traction drive torque demand, AC traction drive speed, and variouselectrical characteristics of the energy storage units, such as SOC,voltage levels, state of health, and temperature. For example, suchdynamic control enables multi-channel bi-directional boost converter 106to independently control the amount of energy supplied by highspecific-power energy storage device 104 and/or energy battery 102during typical vehicle acceleration. Likewise, during deceleration,multi-channel bi-directional boost converter 106 operates to control theamount of regenerated energy provided to energy storage device 112, highspecific-power energy storage device 104, and/or energy battery 102 tomaximize the overall charge acceptance of the system. Such dynamiccontrol greatly improves the overall efficiency of system 100.

FIG. 2 illustrates another embodiment of the invention. Propulsionsystem 200 shown in FIG. 2 includes components similar to componentsshown in system 100 of FIG. 1, and thus numbers used to indicatecomponents in FIG. 1 will also be used to indicate similar components inFIG. 2. As shown, system 200 includes the components of system 100,along with additional components such as a plurality of voltage sensors126, a current sensor 128, a pre-charge circuit 132, and a VehicleSystem Control (VSC) 134. Pre-charge circuit 132 acts to provide aninitial pre-charge to a DC link filter capacitor 218 associated withDC-AC Inverter 114, plus other filter and energy storage capacitorsassociated with the EMS during vehicle start-up. Commands for such avehicle start-up come from VCS 134, which receives operator inputs suchas start-up, acceleration, and deceleration, and controls the operationof system 200 accordingly. It is to be understood that energy battery102, high specific-power energy storage device 104, multi-channelbi-directional boost converter 106, and energy storage device 112 ofsystem 200 may be operated similarly to that described above withrespect to system 100. Alternatively, energy battery 102 may be removedfrom the first energy storage system, thereby making high specific-powerenergy storage device 104 the only energy storage device on low-voltageside 202 of system 200. Such a configuration would primarily be used inhybrid-electric drive-train configurations, wherein a heat engine (notshown) could supplement the energy provided via the first energy storagesystem and the second energy storage system.

FIG. 3 illustrates yet another embodiment of the invention. Propulsionsystem 300 shown in FIG. 3 includes components similar to componentsshown in systems 100 and 200 of FIGS. 1 and 2, and thus numbers used toindicate components in FIGS. 1 and 2 will also be used to indicatesimilar components in FIG. 3. As shown, system 300 includes an auxiliarypower unit 302 on low-voltage side 202 of multi-channel bi-directionalboost converter 106. Auxiliary power unit 302 comprises a heat engine136, an alternator 138, and a rectifier 140. Auxiliary power unit 302 ofsystem 300 also includes a plug-in electrical system comprising an ACplug 142, an isolation transformer 144, a Ground Fault CurrentInterrupter (GFI) 146, and a rectifier 148. The output of rectifier 140is coupled to bus 222 such that energy produced by heat engine 136 andalternator 138 may supplement the energy provided by high specific-powerenergy storage device 104, and/or energy battery 102. Furthermore, whenheat engine 136 is operating, energy battery 102, high specific-powerenergy storage device 104, and energy storage device 112 selectively maybe recharged using energy provided via heat engine 136, alternator 138,and rectifier 140. Control of the current, voltage, and power iscontrolled during recharge operation via VSC 134 and the EMS.

Alternatively, when energy battery 102, high specific-power energystorage device 104, and energy storage device 112 are not being used tooperate motor 116, AC plug 142 may be coupled to an external electricalpower source (i.e., the utility grid) to supply energy through rectifier148 to the energy storage devices 102, 104, 112 in system 300. Theoutput 304 of rectifier 148 is coupled through an inductor to a separatechannel (e.g., channel “c”) of multi-channel bi-directional boostconverter 106 such that voltage, current, and power from the externalelectrical power source is controlled and is capable of being providedto any of energy battery 102, high specific-power energy storage device104, and energy storage device 112 in system 300. In FIG. 3, a contactor130 acts to prevent enablement of DC-AC inverter 114 during charging ofenergy battery 102, high specific-power energy storage device 104, andenergy storage device 112 when the system is plugged into an electricutility interface via AC plug 142. While contactor 130 is shown betweenenergy storage device 112 and DC-AC inverter 114, contactor 130 may belocated elsewhere in system 300, including each phase on AC motor 116.Accordingly, when incorporated into a vehicle, system 300 shown in FIG.3 is not only capable of energy recharge via heat engine 136 while underoperation, but can also be recharged when the vehicle is not in use.

Unlike systems 100 and 200 respectively shown in FIGS. 1 and 2, system300 illustrated in FIG. 3 is shown without a unidirectional couplingdevice (e.g., a diode) between energy battery 102 and highspecific-power energy storage device 104. Without such a unidirectionalcoupling device, high specific-power energy storage device 104 may bedischarged to a value substantially lower than the voltage of energybattery 102. In this way, the efficiency of system 300 during operationof AC motor 116 at low speed and low power is greatly improved.

FIG. 4 illustrates yet another embodiment of the invention. Propulsionsystem 400 shown in FIG. 4 includes a number of components similar tocomponents shown in systems 100, 200, and 300 of FIGS. 1-3, and thusnumbers used to indicate components in FIGS. 1-3 will also be used toindicate similar components in FIG. 4. According to various embodiments,propulsion system 400 may be configured as an electric propulsionsystem, a hybrid propulsion system, or a plug-in hybrid-electricpropulsion system, as examples.

As illustrated, propulsion system 400 includes, in part, a first energystorage system 402, a bi-directional boost converter 404, and a secondenergy storage system 406. Propulsion system 400 is configured such thatfirst energy storage system 402 has a lower voltage than second energystorage system 406. In one embodiment, the voltage of second energystorage system 406 is a factor of three or greater higher than thevoltage of first energy storage system 402.

First energy storage system 402, which is electrically coupled to alow-voltage side 408 of bi-directional boost converter 404, comprises ahigh specific-energy storage device having a relatively low specificpower as compared with second energy storage system 406. As used herein,low specific power describes an energy storage device demonstrated toachieve a specific power on the order of 200 W/kg or lower. In oneembodiment, first energy storage system 402 has a relatively highresistivity and impedance as compared with second energy storage system406. In another embodiment, the relatively low specific power of energystorage system 402 may be due to an imbalance of the individual batterycells comprising the energy storage system. In one embodiment, firstenergy storage system 402 is a low-cost lithium ion battery.Alternatively, first energy storage system 402 may be a sodium metalhalide battery, a sodium sulfur battery, a nickel metal halide battery,a lead acid battery, and the like.

Second energy storage system 406 comprises a high specific-power energystorage device and is electrically coupled to a high-voltage side 410 ofbi-directional boost converter 404. Second energy storage system 406 maybe, for example, an ultracapacitor having multiple capacitor cellscoupled to one another, where the capacitor cells may each have acapacitance that is greater than 500 Farads, similar to high-specificpower energy storage device 104 of FIG. 1. In an alternative embodiment,second energy storage system 406 is a high power battery having aspecific-power of 350 W/kg or greater.

As shown in FIG. 4, first energy storage system 402 and second energystorage system 406 are coupled together in series to DC link 412.Specifically, a negative terminal 414 of first energy storage system 402is connected to a negative side of DC link 416, and a positive terminal418 of first energy storage system 402 is coupled in series with anegative terminal 420 of second energy storage system 406 via link 422.As shown, link 422 bypasses bi-directional boost converter 404 to enablethe voltage outputs of energy storage systems 402, 406 to be summed on apositive side of DC link 412.

By connecting energy storage systems 402, 406 in series, the powersharing between energy storage systems 402, 406 is a function of therelative voltages of the two energy storage systems, rather than beingbased on the relative resistance of the energy storage systems, as isthe case in propulsion systems where the energy storage units areconfigured in a hard parallel arrangement. In other words, the power outof each energy storage system 402, 406 is a function of the voltage ofthe respective energy storage system 402, 406 as a result of the seriesconfiguration. Because the series connection of energy storage systems402, 406 allows the relative voltages of the two systems to be summed,energy storage systems 402, 406 may be sized to have lower voltages thana propulsion system with a parallel configuration with a comparableoverall voltage output. Thus, each energy storage system 402, 406 may beconstructed having fewer series-connected cells as compared with aconventional traction battery comprised of a large number of seriesconnected battery cells coupled across the DC link or load. As such, theseries connection of energy storage systems 402, 406 increases energystorage system reliability and reduces balancing issues and costs foreach energy storage system 402, 406.

The series connection also passively reduces the power demand from firstenergy storage system 402 by placing it in series with the highervoltage second energy storage system 406. As a result of the seriesconfiguration and relative voltages of energy storage systems 402, 406,the majority of the link power comes from second energy storage system406 under positive power demand (e.g., acceleration). Under negativepower demand (e.g., regenerative braking), on the other hand, most ofthe link power charges second energy storage system 406. Thus, only asmall fraction of the link power is available to charge first energystorage system 402. High power regenerative braking energy is capturedin energy storage systems 402, 406 and can be used in subsequentaccelerations. Because the second energy storage system 406 provides themajority of the power demand during acceleration, receives the majorityof the link power during regenerative braking, and operates at amid-range SOC, the overall power capability of propulsion system 400 ishigh for both discharge (acceleration) and charge (regenerativebraking).

During operation of propulsion system 400, bi-directional boostconverter 404 is operable as an Energy Management System (EMS) toadaptively control energy transfer through converter 404 for chargingsecond energy storage system 406. The EMS operates to maintain the stateof charge (SOC) and voltage range of second energy storage system 406 tomaintain adequate power for both discharge (acceleration) and charge(regenerative braking) The EMS operates with adaptive control that maybe programmed to vary with vehicle operating characteristics, such as,for example, vehicle speed, traction motor(s) speed, and motor torque.The adaptive control of EMS may also be programmed to vary based on theelectrical characteristics of energy storage systems 402, 406, includingSOC, voltage levels, state of health, and temperature, and the like. Forexample, when the SOC of second energy storage system 406 becomes toolow, such dynamic control boosts voltage supplied by first energystorage system 402 and transmits the boosted voltage to second energystorage system 406 to maintain the SOC of second energy storage system406. In one embodiment, the EMS of bi-directional boost converter 404operates to maintain an SOC of second energy storage system 406 betweena predetermined lower threshold value, such as, for example 40 percent,and a predetermined upper threshold value, such as, for example 70percent.

During certain motoring events, motoring power/current selectivelybypasses bi-directional boost converter 404 to utilize the benefits ofthe series connection of energy storage systems 402, 406. During heavyvehicle acceleration, for example, the combined voltage of energystorage devices 402, 406 may be used to provide sufficient voltage andpower to AC motor 116 without incurring losses related to passingcurrent through bi-directional boost converter 404. During regenerativebraking events, regenerative energy can be efficiently stored in energystorage devices 402, 406 again without incurring the losses associatedwith the limitations of boost converter 404, as series link 422 enablescurrent to bypass boost converter 404. The energy stored in energystorage devices 402, 406 can then be used for subsequent accelerations,which again improves the overall efficiency of the entire propulsionsystem 400.

The series connection of first energy storage system 402 and secondenergy storage system 406 has the benefit of reducing stress on theenergy storage systems 402, 406, thereby improving battery life, ascompared with a hard parallel configuration of energy storage systems402, 406. For example, in a hard parallel configuration with energystorage system 402 configured as a low specific power battery and energystorage system 406 configured as an ultracapacitor, a high peak currentflows from low specific power battery 402 to ultracapacitor 406 after anacceleration event to recharge ultracapacitor 406.

An additional benefit of the series configuration of propulsion system400 is an improved efficiency as compared with a parallel configuration.The series arrangement of energy storage systems 402, 406 increasessystem efficiency by reducing the power demand on first energy storagesystem 402, which provides improved vehicle range due to a higheruseable capacity of energy storage systems 402, 406. Further, becausefirst energy storage system 402 only accounts for a small fraction ofthe total DC link voltage, the DC link voltage drop during accelerationand the DC link voltage rise during deceleration is much lower than in aparallel energy storage configuration due to the high resistance offirst energy storage system 402.

Optionally, propulsion system 400 includes a second bi-directionalconverter 424 (shown in phantom) to control the DC link voltage. Addingsecond bi-directional converter 424 decouples the voltages of firstenergy storage system 402 and second energy storage system 406 from DClink 412, which provides a number of benefits, including greaterflexibility in sizing first and second energy storage systems 402, 406.

The dual bi-directional boost converter arrangement also offsets thenegative aspects of the higher resistance characterized by the use of alow specific power energy storage system. The use of the secondbi-directional converter 424 provides DC link voltage control, whichenables propulsion system 400 to be constructed with a smaller and lessexpensive motor and inverter and allows better control of motor power.Additionally, the dual boost converter arrangement also may result inlower current boost converters than a propulsion system including amulti-channel converter.

Therefore, according to one embodiment of the invention, a propulsionsystem includes an electric drive and a first energy storage systemelectrically coupled to the electric drive through a DC link, the firstenergy storage system comprising a high specific-energy storage device.The propulsion system also includes a second energy storage systemelectrically coupled to the first energy storage system in a seriesconnection, the second energy storage system comprising a highspecific-power storage device. The propulsion system further includes afirst bi-directional boost converter electrically coupled to the firstand second energy storage systems such that a terminal of the firstenergy storage system is electrically coupled to a low voltage side ofthe first bi-directional boost converter and a first terminal of thesecond energy storage system is coupled to a high voltage side of thefirst bi-directional boost converter. The series connection bypasses thefirst bi-directional boost converter.

According to another embodiment of the invention, a method of assemblinga control system for an electric drive includes providing a firstbi-directional boost converter and coupling a first terminal of a highspecific-energy storage device to a low-voltage side of the firstbi-directional boost converter. The method also includes coupling afirst terminal of a high specific-power storage device to a high-voltageside of the first bi-directional boost converter, and coupling the highspecific-energy storage device in series with the high specific-powerstorage device such that the first terminal of the high specific-energystorage device is electrically coupled to a second terminal of the highspecific-power storage device.

According to yet another embodiment of the invention, an energy storagearrangement for an electrically powered system includes a firstbi-directional boost converter and a first energy storage systemelectrically coupled to a first side of the first bi-directional boostconverter, the first energy storage system comprising a highspecific-energy storage device. The energy storage arrangement alsoincludes a second energy storage system electrically coupled to a secondside of the first bi-directional boost converter, the second energystorage system comprising a high specific-power storage device. Further,the energy storage arrangement includes an electrical link forming aseries connection between a positive terminal of the first energystorage system and a negative terminal of the second energy storagesystem, wherein the electrical link bypasses the first bi-directionalboost converter.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims.

What is claimed is:
 1. A propulsion system comprising: an electricdrive; a first energy storage system electrically coupled to theelectric drive through a direct current (DC) link, the first energystorage system comprising a high specific-energy storage device; asecond energy storage system electrically coupled to the first energystorage system in a series connection, the second energy storage systemcomprising a high specific-power storage device; and a firstbi-directional boost converter electrically coupled to the first andsecond energy storage systems such that a terminal of the first energystorage system is electrically coupled to a low voltage side of thefirst bi-directional boost converter and a first terminal of the secondenergy storage system is coupled to a high voltage side of the firstbi-directional boost converter; and wherein the series connectionbypasses the first bi-directional boost converter.
 2. The propulsionsystem of claim 1 wherein the first energy storage system comprises alow specific power battery.
 3. The propulsion system of claim 2 whereinthe first energy storage system comprises a lithium ion battery.
 4. Thepropulsion system of claim 1 wherein the second energy storage systemcomprises a power battery.
 5. The propulsion system of claim 1 whereinthe second energy storage system comprises an ultracapacitor.
 6. Thepropulsion system of claim 1 wherein the first bi-directional boostconverter is configured to control an amount of energy transferred fromthe first energy storage system to the second energy storage systemthrough the first bi-directional boost converter to maintain astate-of-charge (SOC) of the second energy storage system.
 7. Thepropulsion system of claim 1 further comprising a second bi-directionalboost converter, wherein a low voltage side of the second bi-directionalboost converter is electrically coupled to the first terminal of thesecond energy storage system, and wherein a high voltage side of thesecond bi-directional boost converter is electrically coupled to the DClink.
 8. The propulsion system of claim 7 wherein a power rating of thefirst bi-directional boost converter is lower than a power rating of thesecond bi-directional boost converter.
 9. A method of assembling acontrol system for an electric drive comprising: providing a firstbi-directional boost converter; coupling a first terminal of a highspecific-energy storage device to a low-voltage side of the firstbi-directional boost converter; coupling a first terminal of a highspecific-power storage device to a high-voltage side of the firstbi-directional boost converter; and coupling the high specific-energystorage device in series with the high specific-power storage devicesuch that the first terminal of the high specific-energy storage deviceis electrically coupled to a second terminal of the high specific-powerstorage device.
 10. The method of claim 9 wherein coupling the highspecific-energy storage device to the high specific-power storage devicefurther comprises providing an electrical link between the highspecific-energy storage device and the high specific-power storagedevice that bypasses the first bi-directional boost converter.
 11. Themethod of claim 9 further comprising: providing a second bi-directionalboost converter having a power rating that is higher than a power ratingof the first bi-directional boost converter; and coupling the firstterminal of a high specific-power storage device to a low voltage sideof the second bi-directional boost converter.
 12. An energy storagearrangement for an electrically powered system, the arrangementcomprising: a first bi-directional boost converter; a first energystorage system electrically coupled to a first side of the firstbi-directional boost converter, the first energy storage systemcomprising a high specific-energy storage device; a second energystorage system electrically coupled to a second side of the firstbi-directional boost converter, the second energy storage systemcomprising a high specific-power storage device; an electrical linkforming a series connection between a positive terminal of the firstenergy storage system and a negative terminal of the second energystorage system, wherein the electrical link bypasses the firstbi-directional boost converter.
 13. The energy storage arrangement ofclaim 12 wherein the first energy storage system comprises a lowspecific power battery.
 14. The energy storage arrangement of claim 12wherein the second energy storage system comprises a power battery. 15.The energy storage arrangement of claim 12 wherein the second energystorage system comprises an ultracapacitor.
 16. The energy storagearrangement of claim 12 wherein the first bi-directional boost convertercomprises an energy management system programmed to selectively transmita charging voltage from the first energy storage system to the secondenergy storage system.
 17. The energy storage arrangement of claim 16wherein the energy management system is programmed to maintain astate-of-charge (SOC) of the second energy storage system between apredetermined lower threshold and a predetermined upper threshold. 18.The energy storage arrangement of claim 12 wherein a voltage of thefirst energy storage system is lower than a voltage of the second energystorage system.
 19. The energy storage arrangement of claim 12 furthercomprising a second bi-directional boost converter having a low voltageside electrically coupled to a positive terminal of the second energystorage system.
 20. The energy storage arrangement of claim 19 wherein apower rating of the second bi-directional boost converter is higher thana power rating of the first bi-directional boost converter.